I have the exact same problem - even the route of my previous workouts display only the starting location. It seems to be a bigger SW problem in the Apple HealthKit database on iOS 14, since other users are reporting this as well...
I‘ve tried unpairing / re-pairing, resetting / restoring (AW + iPhone) and setting up as new (AW + iPhone) - nothing seems to work.
I guess we need to wait until Apple fixes this issue with a future SW update.
In the meantime we need to address this issue in a single community thread - this one has the most replies so far (please follow up there):
https://discussions.apple.com/thread/251806066
[AW S3 on watchOS 7 + iPhone X on iOS 14]